Institute for Energy Technology in Norway (IFE) has, since the nineteen sixties, worked with development of tracer technology for industrial applications. Since the beginning of the nineteen eighties the focus has been on the oil and gas industry. Many passive inter-well (well-to-well) tracers have been tested and qualified, and in recent years, some families of partitioning tracers have also been tested in laboratory and field experiments. The laboratory tests include flooding experiments at simulated reservoir conditions using sand-packed columns containing crude oil at residual oil saturation. The tracer candidates are also tested for thermal stability, and adsorption in closed vials with anaerobe atmosphere, with and without rock materials present.
Partitioning tracers are simultaneously injected with a passive tracer as a pulse in partitioning inter-well tracer tests (PITT). Due to the solubility of the partitioning tracers in the oil phase, these tracers will move more slowly through the reservoir than the non-retained passive tracer. When the oil/water partition coefficient for the partitioning tracer is known, the residual oil saturation can be calculated when the difference in migration times for the passive and the partitioning tracers have been measured.
The Partitioning Inter-well Tracer Test (PITT) technology has potential to become a standard method for identifying enhanced oil recovery (EOR) targets, and for evaluation of performance of EOR operations. PITTs have successfully been applied in some oil fields producing at marginal oil rates. The Partitioning Inter-well Tracers Tests to determine residual oil saturation is based on chromatographic separation of tracers in the reservoir [1],[2],[3]. Tracers with different oil/water partition coefficients are introduced with injection water, and samples of water are collected from the production stream for analysis. The tracers will move through the reservoir at different velocities depending on the partition coefficients and the oil saturation in the volume between injection and production wells. The oil saturation for a field with negligible oil flow rates compared to the water flow rates (a field close to residual oil saturation) can be described by chromatographic theory and calculated from the following equation:
                    S        =                                            T              R                        -                          T              R              W                                                          T              R                        +                                          T                R                W                            ⁡                              (                                  K                  -                  1                                )                                                                        (                  equation          ⁢                                          ⁢          1                )            
Here TR and TRW are the retention times of the partitioning and passive water tracer, respectively, S is the residual oil saturation, and K is the partition coefficient of the partitioning tracer.
If the partition coefficient is known, the residual oil saturation can be calculated from the measured difference in the arrival times between a non-partitioning (passive) and a partitioning tracer. This equation is only valid as long as the tracers do not interact with the rock material. Different groups of chemicals have been tested for application as partitioning tracers. Important parameters are the partition coefficient, the thermal stability, the absence of adsorption to rock materials and the analytical detectability.
Certain compounds, such as alcohols, have been used as partitioning tracers to estimate amounts of non-aqueous phase liquid in porous media and remaining oil in the swept area between wells (e.g. McClesky sandstone field test, Landmark method, Leduc test, Ranger field test, [5]). However, many alcohols are naturally present in oil reservoirs, making them difficult to detect at low level and/or distinguish from naturally occurring compounds. Furthermore, while radiolabeled compounds could be used and detected with high sensitivity, such compounds should be avoided due to regulatory restrictions in many areas.
In order to be effective as a partitioning tracer, a compound must display certain key properties for effective function. In particular, an effective partitioning tracer should display an appropriate partition coefficient, should be stable to the temperature conditions of the reservoir, should be environmentally acceptable, should not interact with rock and other material of the oil well and oil field, or should do so in an insignificant or predictable way, and/or should be detectable at low level. Suitable compounds would also advantageously be distinct from the compounds found naturally in oil reserves, such that injected compound can be identified as such down to a low level.
It would be of considerable value to provide new oil field tracer compounds which were not naturally present in that environment and which show partitioning between oil and aqueous phases. Such compounds would advantageously be environmentally acceptable, stable to oil reservoir conditions, show little, or preferably no interaction with oil field materials, have predictable partition to the oil phase and/or be detectable at low levels.